1. Field of the Invention
The present invention relates to a liquid crystal display apparatus for displaying images by modulating a polarization state of linearly-polarized light, such as a liquid crystal television, a liquid crystal display for computers, and a PALC (Plasma Addressed Liquid Crystal) display, and a backlight for use therein.
2. Description of the Related Art
Recently, liquid crystal display apparatuses, more particularly those incorporating color display elements, have been remarkably improved, and their market has been steadily growing because they can save the installation space by virtue of the small thickness and can also save power.
The liquid crystal display apparatus comprises an element (liquid crystal panel) including a liquid crystal layer interposed between a pair of glass substrates, and polarizing plates which are attached on opposite sides of the element (the front and rear sides; light passes through the panel from the rear side to the front side). The polarization state of linearly-polarized light incident upon a liquid crystal panel is modulated by the liquid crystal layer, thereby displaying images.
A transmission-type liquid crystal display apparatus, which is currently dominating among others, requires illumination from the rear thereof. Therefore, a backlight is an essential device to the transmission-type liquid crystal display apparatus.
A conventional backlight emits randomly-polarized light having non-uniform polarization directions. Approximately half of the incident light is absorbed by a polarizing plate on the rear side of the liquid crystal panel, which poses a problem that the light efficiency is thus reduced and it is not possible to achieve high brightness. In mobile personal computers which are basically driven by batteries and PALCs which are expected to be provided in large sizes, i.e., from 40 to 60 inches, reduction of power consumed by the backlight has had a high priority and it has thus been difficult to attain high brightness. Moreover, light absorbed by the polarizing plate on the rear side is converted to heat, increasing the temperature of the liquid crystal display apparatus and thus reducing the reliability thereof.
In order to solve the above-described problems, a liquid crystal display apparatus having a configuration as shown in FIG. 4 has been recently adopted, whose brightness can be increased with polarization-based techniques.
In such a liquid crystal display apparatus, a fluorescent lamp 4, or a light source, is provided at an end of a light guide plate 7, and a polarization reflection film 21 is provided between a diffusing plate 11 and a liquid crystal panel 3. Random light emitted from the light source 4 enters the light guide plate 7, and is reflected off a reflection pattern 12 formed on the lower side of the light guide plate 7. A portion of the reflected light which is polarized in the direction of a transmission axis of the polarization reflection film 21 (indicated by a dashed line in FIG. 4) is transmitted through the polarization reflection film 21 and reaches the liquid crystal panel 3 substantially as it is. Another portion of the reflected light which is polarized in the direction orthogonal to the transmission axis of the polarization reflection film 21 (i.e., along the reflection axis thereof) (indicated by a solid line in FIG. 4) is reflected off the polarization reflection film 21, and returns toward the lower side of the light guide plate 7. The light traveling toward the lower side of the light guide plate 7 has its polarization direction rotated while passing through the diffusing plate 11 or while being reflected again off the reflection pattern 12 on the lower side of the light guide plate 7. Because of this rotation effect, a portion of the return light now has a polarization direction such that it can pass through the polarization reflection film 21. The remaining portion of the return light is again reflected off the polarization reflection film 21 and returns toward the lower side of the light guide plate 7 to have its polarization direction rotated again. As a result of this repeated process, the polarization direction of light reaching the liquid crystal panel 3 can be aligned with the transmission axis of the polarization reflection film 21. If the transmission axis of the polarization reflection film 21 is aligned with the transmission axis of the polarizing plate 3a provided on the rear side of the liquid crystal panel 3, the light efficiency can be improved. Theoretically, the light efficiency can be improved by a factor of about 2, but empirically the factor is around 1.6.
According to the above-described liquid crystal display apparatus, the backlight efficiency can be improved. In the present state of the art, however, a polarization reflection film is difficult to mass-produce and thus is very expensive, which poses a problem that it is difficult to adopt the film for PALCs which are expected to be provided in large sizes, i.e., from 40 to 60 inches.
A PBS (Polarization Beam Splitter) has a similar function as that of the polarization reflection film. Japanese Laid-Open Publication No. 6-160840 discloses a liquid crystal display apparatus including the PBS as shown in FIG. 5.
In the liquid crystal display, a PBS 9 is provided between a light source 4 and an end face of a light guide plate 7. The PBS 9 splits random light from the light source 4 into two light waves with different polarization directions (P-polarized light and S-polarized light) before the light enters the light guide plate 7. One of the light waves (e.g., P-polarized light, as indicated by dashed lines in FIG. 5) enters the Light guide plate 7. The polarization direction of the other one of the light waves (e.g., S-polarized light, as indicated by solid lines in FIG. 5) is changed and the light wave is converted into P-polarized light through a 1/2 phase plate 10 which is provided at an end face of the light guide plate 7.
The PBS can split incident light into P-polarized light and S-polarized light only for angles of incidence from about -10.degree. to about +20.degree.. Due to the narrow angle range, the PBS is not suitable for splitting light from a diffused light source.